Interpretive Summary: In order to determine metabolizable energy or amino acid digestibility, either conventional diets (corn and soybean meal based) or semi-purified (cornstarch-dextrose) diets are commonly used to evaluate feedstuffs in bioassays with poultry. Values obtained from these bioassays are then used in commercial least-cost formulation in order to maximize nutrient efficiency and decrease cost. Bioassays used to determine amino acid digestibility commonly use semi-purified diets but there has been question that they may not accurately measure the true nutrient content of the feedstuff based on rate of passage. Rate of passage (ROP) is the speed in which feed or diets move through the gastrointestinal tract. Therefore, the objectives of this study were to determine the rate of passage of corn and soybean meal based diets and diets comprised of cornstarch and dextrose. Four dietary treatments were formulated. Two diets were corn and soybean meal base supplement with either meat and bone meal (MBM) or distiller’s dried grains with solubles (DDGS). The last two diets were formulated to be semi-purified with the same MBM and DDGS samples. Broilers were fed all four diets with markers included to measure ROP and excreta and ileal digesta were collected to calculate retention times in the broiler digestive tract. Results of this study indicate that broilers fed MBM based diets had no difference in ROP. However, broiler fed a semi-purified diet of DDGs had a faster ROP than corn and soybean meal based DDGS diets. It was concluded that the ROP of semi-purified diets may differ from corn soybean meal based diets. This may be dependent on the ingredient being evaluated and the effect of ROP on broiler growth performance has yet to be enumerated.

Technical Abstract:
This experiment evaluated rate of passage (ROP) and standardized ileal amino acid digestibility (SIAAD) of 4 diets varying in ingredient composition fed to broilers from 14 to 22 d of age. Two hundred and eighty-eight Ross × Ross 708 chicks (12 birds per pen; 0.45 m2 per bird) were randomly assigned to 24 pens (6 replicate pens per treatment) at 1 d of age. Experimental diets were: 1) corn-soybean meal-based (CSM) diet containing porcine meat and bone meal (MBM) (5% inclusion), 2) CSM diet containing distiller’s dried grains with solubles (DDGS) (5% inclusion), 3) semi-purified (SP) diet containing MBM (38% inclusion), and 4) SP diet containing DDGS (76% inclusion). Diets were formulated to contain 20% CP and were adequate for vitamins and minerals. Experimental diets were provided from d 14 to 22. On d 18, a total excreta collection was conducted every h for 12 h from 8:00 to 19:00 h to determine ROP, and SIAAD was determined on d 22. Time of 1% TiO2 excretion (T1), 50% TiO2 excretion (T50), and mean retention time (MRT) were used to express ROP. The SP-MBM diet reached T1 (P < 0.05) faster than the SP-DDGS or 2 CSM diets. However, T50 indicated a faster (P < 0.05) ROP for SP-DDGS than the 2 CSM diets, and no significant difference was observed between the 2 SP diets. The MRT of the SP-DDGS diet (5.13 h) was less (P < 0.05) than the MRT of the SP-MBM, CSM-MBM, and CSM-DDGS diets, and resulted in values of 5.48, 5.62 and 5.58 h, respectively. In general, the SIAAD values were higher (P < 0.05) for the 2 CSM diets than for SP diets. Except for His, no statistical differences were observed between the SIAAD of the 2 CSM diets. Comparing the 2 SP diets, SIAAD was usually similar or higher (P < 0.05) for SP-DDGS, except for His, Lys, and Gly, which were higher (P < 0.05) for SP-MBM. Based on T50 and MRT, semi-purified diets containing DDGS had a faster ROP in broilers than CSM diets or SP diets containing MBM.